At present, thin film transistors (hereinafter referred to as “TFTs”) are suitably used for driving elements in such devices as active matrix-type liquid crystal displays. Various configurations have been proposed as the configuration of the TFT, but a basic configuration is as follows; electric current flowing between a source electrode and a drain electrode, which are provided in contact with a semiconductor layer, is controlled by a voltage applied to a gate electrode (in other words, by electric field generated by the applied voltage), which is insulated from the semiconductor layer by an insulating layer. Examples of the semiconductor materials that have currently been utilized for the semiconductor layer that constitutes the TFT include amorphous silicon and low-temperature polysilicon, which are inferior to crystalline silicon in terms of performance but are relatively low in cost. Examples of the insulative materials that have currently been utilized for the insulating layer on which the gate electrode is provided include silicon oxide and silicon nitride. The manufacturing processes of the TFTs that use these semiconductor materials and insulative materials, however, require a large-scale system for a plasma CVD method or the like, or a thin film-controlling system for high-precision processing. Therefore, manufacturing cost of the TFTs is high. Moreover, the just-mentioned manufacturing processes generally involve a process with a temperature exceeding 350° C., and therefore impose restrictions on the substrate materials that can be used.
In recent years, organic semiconductors composed of organic compounds have attracted attention as semiconductor materials that can be utilized for TFTs. In contrast to the cases that use inorganic-based semiconductors, such as the above-mentioned amorphous silicon and low-temperature polysilicon, the organic semiconductors are capable of forming the semiconductor layer through such manufacturing processes as spin coating, ink jet printing, and dip coating, which are low-cost and low-temperature processes. Therefore, cost reduction in the manufacturing cost of TFTs is possible, and moreover, the restrictions to the usable substrate materials etc. are eliminated. Furthermore, since the just-mentioned low-cost processes and low-temperature processes are applicable, TFT fabrication on flexible substrates and large-area substrates can be realized, which is expected to widen the applications to large-screen displays, sheet-like or paper-like displays, wireless ID tags, and so forth. Nevertheless, the organic semiconductors reported to date have lower carrier mobilities than those of the inorganic-based semiconductors. Accordingly, various attempts have been made to achieve carrier mobility comparable to that of amorphous silicon.
Among the organic semiconductors, π-conjugated organic semiconductor is made of an organic compound composed of a molecular structure having a π-conjugated double bond. It is believed that the semiconductor properties are obtained due to the valence band, the conduction band, and the band gap therebetween, that are formed due to the overlap of π orbitals in the π-conjugated double bonds. In an aggregate of π-conjugated organic semiconductor molecules, electrical conductions are as follows, in descending order of ease of the electrical conduction; electrical conduction along the main chain direction in the molecules, electrical conduction making use of the overlap of π orbitals of the adjacent molecules, and electrical conduction originating from electron hopping between the molecules. Therefore, in order to improve the carrier mobility in the π-conjugated organic semiconductor molecules, an issue is how to achieve a configuration that can make use of an electrical conduction that is more effective in terms of the ease of electrical conduction among the electrical conductions. Accordingly, the method of controlling the orientation of the molecules to be in a certain direction has been adopted as a method for lessening the electrical conduction due to electron hopping between the molecules. Specific methods of the orienting that have been disclosed include a method in which a polysilane thin film is oriented using the Langmuir-Blodgett technique (LB technique) or a drawing technique (for example, Japanese Unexamined Patent Publication No. H05-275695). Another method disclosed is a method in which polytetrafluoroethylene is pressed onto a substrate with a certain pressure and is slid to form an orientation, and a oligothiophene compound is brought into contact with the upper surface of the orientation-formed polytetrafluoroethylene film to effect an orientation-deposition (for example, Japanese Unexamined Patent Publication No. H07-206599). Also disclosed is a method in which π-conjugated oligomer molecules are orientation-grown using a hot wall epitaxy method (for example, Japanese Unexamined Patent Publication: Japanese Unexamined Patent Publication No. 2002-270621). Using these methods of orienting can minimize the electrical conduction due to electron hopping between molecules.
In addition, methods for further improving carrier mobility have been proposed, including a method of controlling the orientation direction of π-conjugated organic semiconductor molecules to be parallel to the perpendicular line connecting the source electrode and the drain electrode in the TFT, to thereby attempt to make use of the electrical conduction along the main chain direction in the molecules effectively (for example, Japanese Unexamined Patent Publication No. 5-275695 and Published Japanese Translation of PCT Application No. 2003-502874), and a method of controlling the orientation direction of π-conjugated organic semiconductor molecules to be perpendicular to the perpendicular line connecting the source electrode and the drain electrode in the TFT, to thereby attempt to make use of the electrical conduction utilizing the overlap of the π orbitals of the adjacent molecules effectively (for example, Japanese Unexamined Patent Publication No. 9-116163).
In the method of controlling the orientation direction of the π-conjugated organic semiconductor molecules to be parallel to the linear line connecting the source electrode and the drain electrode of a TFT and thereby making use of the electrical conduction along the main chain direction of the π-conjugated organic semiconductor molecules to achieve high carrier mobility, the number of times of electron transfers between the π-conjugated organic semiconductor molecules gradually increases as the distance between the source electrode and the drain electrode increases relative to the molecular length of the main chain of the π-conjugated organic semiconductor molecule. In this case, the electron transfer between π-conjugated organic semiconductor molecules that are adjacent to each other perpendicularly to the perpendicular line connecting the source electrode and the drain electrode becomes very difficult because the electrons need to travel in the direction orthogonal to the direction in which electric field is formed between the source electrode and the drain electrode. Consequently, sufficient carrier mobility cannot be obtained even if the orientation direction of the π-conjugated organic semiconductor molecules is controlled to be parallel to the perpendicular line connecting the source electrode and the drain electrode of the TFT, except when using a π-conjugated organic semiconductor molecule having a molecular length much longer than the distance between the source electrode and the drain electrode, and when the distance between the source electrode and the drain electrode is sufficiently shorter than the molecular length of the π-conjugated organic semiconductor molecule.
On the other hand, in the method of controlling the orientation direction of the π-conjugated organic semiconductor molecules to be perpendicular to the perpendicular line connecting the source electrode and the drain electrode of the TFT, and making use of the overlap of π orbitals of the adjacent π-conjugated organic semiconductor molecules to thereby achieve high carrier mobility, the longitudinal axes of the main chains of the π-conjugated organic semiconductor molecules are aligned perpendicular to the perpendicular line connecting the source electrode and the drain electrode; therefore, the electrical conduction along the main chain direction in the π-conjugated organic semiconductor molecule does not serve the purpose, and mostly the electrical conduction making use of the overlap of π orbitals between the π-conjugated organic semiconductor molecules becomes dominant. For this reason, as the distance between the source electrode and the drain electrode increases, the number of times of electron transfer increases proportional to the increase in the distance. In short, there is a limit to the improvement in carrier mobility even if the orientation direction of the π-conjugated organic semiconductor molecules is controlled to be perpendicular to the perpendicular line connecting the source electrode and the drain electrode of the TFT and at the same time the degree of orientation is increased.